Skin Diseases in Horses

A horse's skin is vital to the animal's survival. It serves as its anatomical boundary and as the principal organ of communication between the horse and the environment in which it lives. As is the case with other body components, the skin of a horse is subject to attack on a number of fronts, ranging from infectious bacteria to biting insects.

We will take a look at some of these skin afflictions, but first, let us set the stage by examining the way in which a horse's skin is constructed. There are a number of resources available that provide information on equine skin problems. Two good ones--both were used in the preparation of this article, along with other sources--are Veterinary Notes for Horse Owners, first published in 1877 and revised and updated frequently through the years, and, on the more technical side, The Merck Veterinary Manual.

A horse's skin is its largest body organ, ranging from 12-24% of the animal's weight, depending on age.

The skin consists of various cellular and tissue components. There is an epidermis, an appendageal system, dermis, arrector pili muscle, twitch muscle (panniculus carnosus), and a fatty subcutaneous layer known as the panniculus adiposus.

Anne M. Eberhardt

Common signs of dermatitis are raised or bumpy spots on the skin.

The main activity of the epidermis is to produce two types of protein--keratin and melanin. Keratin is a scleroprotein, which is the principal constituent of the epidermis. (A scleroprotein is a simple protein characterized by its insolubility and fibrous structure, and which usually serves a supportive or protective function in the body.) Melanin is the dark, shapeless pigment of the skin and hair.

Another cell type in the epidermis is the Langerhans' cell, a non-neural cell that is active in the immunologic process and possibly in the regulation of keratin formation.

The most important part of the epidermis is the superficial layer, since much of the functional activity of the skin resides here. The proper functioning of this superficial layer is dependent on the structural arrangement of the keratin it contains and perhaps on its lipids (fats and fatlike substances characterized by being water-insoluble). Lipids have an effect on the waterproofing capability of the skin and the absorption of agents into the body.

This superficial layer, known as the stratum corneum, is continuously shed.

The appendageal system is an outgrowth of the epidermis and consists of hair follicles and sebaceous and apocrine glands. In some animals the hair follicles are complex, but in horses they are quite simple, with a single hair emerging from each pore.

The hair goes through periods of growth and inactivity. Shedding, for example, is a seasonal event and represents the loss of inactive hair as a new growth takes its place.

The remainder of the appendageal system consists of the sebaceous glands and apocrine and eccrine sweat glands. Sebaceous glands secrete sebum (a fatty substance) into the follicles and thus to the epidermal surface. The apocrine sweat glands are present wherever hair follicles exist and play an important role in a horse's thermoregulation. Their ducts enter the follicle just above the entrance of the sebaceous ducts. Eccrine sweat glands are present in limited areas and empty directly onto the skin surface.

The major function of a horse's skin is protection. The hair, first of all, provides mechanical protection and also serves as an important filtering system and insulator. The superficial layer of the epidermis, a highly developed, tough, durable, flexible membrane, serves as a chemical and waterproofing structure. If it were to be removed, the skin would act like a mucous membrane where it could be easily permeated by a variety of agents. It's ability to function properly can be compromised by both disease and injury. The skin also offers protection against irradiation. Ultraviolet light is filtered by the coat, and absorbed by melanin granules in the epidermis and hair.

In the horse, the skin is highly important in thermal regulation. About the only way the horse can dissipate a buildup of internal heat is through sweating.

Skin Diseases

Basically, the skin diseases that afflict horses can be divided into two groups--those which arise from infection, either by germs or parasites, and those due to causes other than infections.

Complicating the issue is the fact that, just like humans, horses can suffer from allergies that produce skin problems.

The inclusive term for inflammation of the skin is dermatitis. It can be produced by many agents, such as external irritants, burns, allergens, and trauma, plus bacterial, viral, parasitic, or fungal infections. It also can be associated with other systemic diseases from which the horse might be suffering.

The most common sign of dermatitis is some form of scratching as the horse rubs the affected body part against fences, walls, or posts. Next comes some swelling and redness, followed by raised or bumpy spots on the skin. As the problem progresses, there might be oozing, crusting, and scaling.

When a dermatitis problem surfaces, the important thing is to determine the underlying cause. Treatment would be vastly different, for example, if the horse were suffering from a fungal infection than it would if the problem were the result of an allergy.

Ringworm

One form of dermatitis that can--and does--afflict horses is ringworm, known scientifically as dermatophytosis. There are many sorts of ringworm, and the horse is susceptible to a number of them. Ringworm infection can be spread directly from horse to horse and by using common grooming tools, saddle pads, harnesses, and saddles. Humans can spread ringworm to horses, and vice versa.

Ringworm actually is an infection of keratinized tissue by one of several types of fungi, collectively known as dermatophytes, thus the scientific name of dermatophytosis.

Under most circumstances, dermatophytes grow only in dead, keratinized tissue. Horses most susceptible are the young and debilitated or animals whose immune systems have been compromised.

Infection begins in a growing hair, eventually causing it to break off just above the skin surface, or in the outer surface of the epidermis. Fungal culture is the most effective and specific means of diagnosis.

The two types of ringworm encountered most commonly in horses are caused by Trichophyton equinum, the primary cause, and Microsporum equinum, which is more prominent in some locales. T. equinum is responsible for a type of ringworm that was introduced during the Second World War and was brought to these shores by imported horses.

This type of ringworm produces skin lesions that appear as small, rounded spots, a quarter of an inch to an inch in diameter. The lesions eventually form blisters and break, leaving scabs. This form of ringworm is easily transmitted to humans and is a notoriously itchy form of the disease.

A variety of trichophyton that also attacks horses is T. mentagrophytes. It can be carried not only by humans and horses, but also by rats and mice, which can infect foodstuffs or wind up in horse mangers or feeding areas.

With this type of ringworm, lesions first make their appearance on the forehead and face, neck, and at the root of the tail, but also can spread to other parts of the body. The oldest part of each lesion is at its center, and fungal growth spreads from the edges. The lesions are grayish in color and form crusts on their surface from which broken off hairs protrude.

Still another form of ringworm is caused by the fungi identified as Microsporon canis, which can be contracted by horses from affected dogs. This type of ringworm can be transmitted from dog to man, from man to horse, and from horse to horse. It is characterized by small, round lesions which are covered with tiny scales.

The incubation period for the fungi after they invade the body ranges from as little as four days to as long as one month.

When ringworm strikes, it is time for immediate treatment of the horse and for disinfecting all grooming equipment. If possible, the horse should be isolated to prevent further spread of the disease. For treatment, many topical medications work well to prevent progression of existing lesions and, at the same, serve as a barrier to contaminating other horses. Thick crusts should be removed gently with a brush and mild soap and the contaminated material burned.

It is recommended that the desired medication be applied to the entire body of the affected animal on a daily basis for five to seven days, then weekly until the infection is controlled.

Available in Europe, but not in the United States and Canada, is a fungal vaccine that has been effective in preventing ringworm.

Ringworm is more common during the winter when animals are stabled, but can crop up at any time.

Rain Rot

Another form of dermatitis that can affect horses is dermatophilosis, more commonly known as rain rot. This troublesome fungal infection is much more common in wet, humid climates. It will crop up frequently in Kentucky and other southern states, for example, but is rarely seen in the Great Plains or Rocky Mountains.

The official name for the fungus is Dermatophilus congolensis. In domestic animals, the condition can affect horses, cattle, sheep, and goats, but is rarely found in pigs, dogs, or cats.

The natural habitat of D. congolensis is unknown, but many researchers believe that it lives in the soil. However, attempts to isolate it from soil have been unsuccessful. It has been isolated only from the skin of various animals and is restricted to the living layers of the epidermis. Infected animals are considered the primary reservoir.

The fungi can live in dormancy within the skin for some time and become active when the skin is compromised in some way, such as prolonged wetting by rain, high humidity, high temperature, and attacks by biting insects. When the infective zoospores reach a skin site where the normal protective barriers of the skin are reduced or deficient, they set up shop and infection is the result.

Zoospores germinate to produce hyphae, threadlike tentacles, which penetrate into the living epidermis and subsequently spread in all directions from the initial attack site. The result is an acute inflammatory reaction.

The good news in the case of an acute infection is that the invasion by the hyphae ceases in two to three weeks and the lesions heal spontaneously. In chronic infections, however, the affected hair follicles and scabs are sites from which intermittent invasions of non-infected hair follicles and epidermis occur. Moisture facilitates release of zoospores from pre-existing lesions and their subsequent penetration of the epidermis to establish new sites of infection. Hot weather and high humidity also can be indirect factors in that a wide variety of biting insects hatch and become active under those conditions, compromising the skin's protective function.

Horse owners whose animals spend most of their time outdoors in the wet, rainy season would do well to examine the animals for rain rot periodically. A visual evaluation won't get the job done, especially during the wet winter months when the horses have full coats of hair. A hands-on examination is the most effective. The infected horse usually will have a series of bumps along the back and croup. When rubbed, the "bumps" might come free in the form of scabs with a small, hairless spot of skin showing. Unlike ringworm, rain rot is not itchy.

While the infection is relatively easy to diagnose in the above manner, a definitive diagnosis can be made only from taking a culture and testing it in a laboratory.

Treatment of infected animals is relatively straightforward. A cure can normally be effected with a single intra-muscular injection of procaine penicillin (22,000 IU/kg) and streptomycin (22 mg/kg).

External treatment with disinfectants that contain a cresol or copper salt base can decrease the spread of infection if applied at times when transmission is likely. Controlling biting insects by applying insecticides also can be effective.

Nature's best cure for rain rot is a warm sun and dry weather.

More Dermatitis

Still another form of dermatitis in horses is known as the Eosinophilic Granuloma Complex. The term is used to describe a group of diseases that affects horses, cats, and dogs, the cause of which is unknown.

In horses, the lesions that result from the disease are nodular, do not ulcerate, and do not itch. They often occur in the saddle area and have a gray-white central core. Both insect bites and trauma have been suggested as causes, but the fact that the lesions can occur in non-contact areas suggests that the cause is multi-faceted.

Horses with multiple lesions can be treated with oral prednisone or prednisolone at 1.1 mg/kg on a daily basis for two to three weeks.

Mange

A skin affliction that has a definite cause is mange. This is a contagious skin disease caused by one of several species of mites. The disease is transmitted when larvae, nymphs, or fertilized females are transferred to a susceptible host. Mange can be transmitted directly from horse to horse or indirectly through contaminated objects that harbor the mites or through contaminated stabling quarters.

Mange is characterized by loss of hair, itching, and intense irritation to the skin. In severe cases, this can lead to debilitation; in drastic instances, it can even lead to death.

The incubation period is two to six weeks and depends on the number of mites transferred, the site of transfer, and host susceptibility. A horse in poor physical condition, for example, would be more susceptible than a horse on a healthy diet and exercise program. Mange was prevalent some years ago, especially in large groups of horses, but is not nearly as common today.

There are four basic types of mange. They are as follows:

Sarcoptic Mange--This is caused by the mite known as Sarcoptes equi, which burrows into the skin. The female mite is the first to attack as she creates tubular burrows in which to lay her eggs. The burrow is created as the female literally eats her way along. As she travels, she leaves behind in the burrow 10 to 40 eggs, along with her excrement. When the young hatch, they compound matters by starting burrows of their own. The burrows of both the female and the young, for the most part, lie very close to the skin surface. The irritation that results from this burrowing is intense, and afflicted horses will be seen to rub almost constantly.

Hair will fall out of the affected patches of skin and scabs will form. The rubbing and scratching of affected horses often cause raw spots on the skin that serve as open doors for the invasion of other harmful agents.

The female mite is a hardy creature that can live in moist situations in tack or in the woodwork of stables. Its greatest enemy is a dry condition.

Treatment of afflicted horses consists of applying mite-killing solutions by spraying, rubbing, or dipping. Toxaphene dip is reported to give control with one application. Lindane can be used as a spray or dip where the use is permitted.

Psoroptic Scapies--This form of mange is a notifiable and quarantinable disease in the countries in which it is found. These mites do not burrow into the skin as the sarcoptes do, but are scavengers on the skin surface. They have piercing mandibles which enable them to penetrate the skin and suck its juices. As a result, they produce a great deal of irritation to the horse, but are easily controlled by applying mite-killing solutions to the skin.

Unfortunately, they, too, have the ability to exist for long periods outside the body in tack, bedding, and woodwork under moist conditions.

These mites are found primarily in sheltered areas of the body, such as under the forelock and mane, at the root of the tail, under the chin, between the hind legs, and in the folds of skin where the forearm joins the chest.

Treatment is the same as that for sarcoptic mange.

Chorioptic Mange--This is also known as leg mange. This variety of mange pretty much confines itself to the cannon bone area, especially on the rear legs. In severe cases, skin lesions can spread to the flanks, shoulder, and neck. The disease is characterized by severe itching, scaling, crusting, and thickening of the skin. This type of mange is seen more often in the winter months than during the summer. Most apt to be afflicted are draft horses with "feathered" lower legs. Treatment is the same as for the two types of mange listed above.

Demodectic Mange--This type is not common. The mites live in the hair follicles and sebaceous glands and produce papules (bumps) and ulcers, particularly around the eyes and on the forehead. Subsequently, the lesions spread to the shoulders and finally over the entire body. It is fortunate that this form of mange is rare because there is no satisfactory treatment.

Lice

In the same general category as mange is pediculosis--the scientific term for infestation by lice. Horses can harbor two species of lice. They are the horse biting louse (Damalinia equi), and the horse sucking louse (Haematopinus asini).

Afflicted horses will show signs of itching by constantly rubbing and biting at affected areas. Lice most often are found in unthrifty horses and quickly add to the poor condition status. In a severe infestation, the horse can lose patches of hair.

Infestations are more apt to occur during the winter when hair coats are long. Lice can be quickly eradicated with spray-on or rub-on insecticides, a variety of which are available commercially.

Insects

Many biting and sucking insects, ranging from stable flies to ticks, also can pose skin problems for horses. Their attack, in essence, is on two fronts. First, they cause extreme irritation as they bite the horse and, second, they open the way for invasion of the skin by other organisms.

Viruses

Skin problems can also be caused by viral invasions. A case in point is papillomatosis--warts. An incidence of warts would be more common in younger animals than in adults (see AAEP AnswerLine in The Horse of September 1996, page 90).

In horses, small, scattered warts can occur on the nose and lips, presumably at the site of abrasions when foals nuzzle each other. Warts in young horses are not uncommon, but normally they will disappear soon and the animal will have developed an immunity against future attacks by the virus.

The "under portions" of the body also can be affected, particularly inside the thighs and on the prepuce. Warm, sunny weather and being turned out on grass facilitate the disappearance of warts.

If warts become a herd problem, vaccination might be an option, but because warts are so species specific, what works for one species might not work for another. The most effective vaccination approach has been with a suspension of ground wart tissue in which the virus has been killed with formaldehyde.

Other Skin Problems

Now for a look at additional afflictions, some of which have causes other than invasion by infection-causing organisms.

Anhidrosis--This problem, also known as dry coat, is pretty much climate-specific. It is found in areas with a hot, humid climate, such as Florida during the summer months. Horses with this affliction, quite simply, can't sweat. And when a horse can't sweat, it loses its thermoregulatory capability. It is a condition that can completely compromise a horse's ability to perform. (See The Horse, July 1997, page 44.)

No one is quite sure what causes anhidrosis, and there is no effective way to treat it, other than to move affected horses to a cooler climate. Quite often, racehorses which suffer from anhidrosis in Florida will show no signs of the affliction when moved to the Midwest during the height of the southern hot weather cycle.

Pityriasis--This is the scientific term for dandruff, a condition in which the skin becomes scaly and the coat dry and dirty. In some instances, the hair will fall out. A lack of grooming and exercise often is the culprit. Good grooming and a balanced diet usually can eliminate the problem.

Pyoderma--A more common name for this condition is acne. It is an infection characterized by the formation of pus. Acne is actually an inflammation of the hair follicles. The cause of the disease is listed as being pyogenic (pus forming) organisms that have gained access to the hair follicles. It will be seen most frequently in parts of the body exposed to friction from saddle or harness. It can also be transmitted via biting flies.

An afflicted animal will show signs of tenderness when the affected body part is touched. When mature, a yellow point appears at the affected spot. Shortly thereafter it will rupture, eliminating pus. Normally, in the wake of recovery, a permanent mark is left, frequently in the form of white hairs.

When pyoderma develops, irritation of the affected area through continued saddling or harnessing should be avoided. Bathing the area in warm antiseptic solutions such as hexachlorophene or providone-iodine can be helpful. The problem can be either superficial or deep and in either case, the recommended form of treatment is administration of antibiotics.

Saddle Sores or Galls--A condition that is somewhat similar to acne or pyoderma involves saddle sores. The areas of skin under the saddle on riding horses and on the shoulder area of driving horses frequently are the sites of skin and soft tissue injury. Clinical signs will vary, depending on the depth of injury as well as complications from secondary infections.

Frequently, the condition starts as an acute inflammation of the hair follicles and progresses to a pus-producing condition. Affected areas might show hair loss and areas that are hot, swollen, and painful. Advanced lesions are termed "galls." In severe cases, abscesses can develop.

Frequently, the cause of saddle sores is ill-fitting tack. When the condition arises, the first line of treatment is absolute rest of the affected parts. During the early stages, astringent packs (Burow's solution or 2% lead acetate) can be helpful. Chronic lesions and those superficially affected can be treated by warm applications and massage with stimulating ointments or with antibiotics.

Needless to say, identifying and eliminating the cause of the problem--ill-fitting tack--is the most important remedy.

Cracked Heel--This is a condition that can be very irritating to the horse and difficult to clear up. It is a form of psoriasis and occurs in the hollow of the pastern. It will be found most frequently in wet, cold weather.

The condition is characterized by the skin in the hollow of the pastern becoming reddened, tender, and scaly. Later, small vesicles (sacs containing liquid) form. When they rupture, a crack appears over the area in which the vesicle formed. If the area is not treated at this point, the crack will deepen and extend and its edges will become thickened and calloused. As the condition worsens, the horse can become lame and will attempt to walk on its toes in an effort to prevent flexing.

In mild cases, the treatment of choice is to wash the affected part with medicated soap and make an effort to keep the horse in an area that is clean and dry. In advanced cases, it might be necessary to apply medication and cover the affected area with a pad and bandage.

Photosensitization--This is a condition in which lightly pigmented skin is hyperreactive to sunlight due to a photodynamic agent in the skin. Horses with pink noses are good candidates for this affliction. Those affected will demonstrate discomfort and will seek to scratch or rub the affected area. A reddening of the skin will develop rapidly and might be followed by swelling and, later, scab formation.

Afflicted horses should be kept under cover, such as in a barn, during the day and allowed to graze in the evening hours. The months of June, July, and August pose the greatest danger to photosensitive horses.

Mane and Tail Eczema--This is a condition that many horsemen have encountered. It is a dermatitis that is attributed to dirt and neglect. It frequently shows up on the tailhead and is called to our attention when the horse backs up to a post and begins rubbing. Later, the skin becomes thickened, hard, and scaly. Washing the tail head or mane area with disinfectant soap can help eradicate the problem.

Alopecia--This is the scientific term used to describe general hair loss. It can occur in horses during pregnancy, lactation, or several weeks after a severe illness or fever. It can also occur as the result of repeated local friction, the continual application of chemicals, such as certain insect repellents to horses with sensitive skin, and as the result of bacterial or dermatophyte infections. Therapy can only be determined after the cause is known.

Cancer--As is the case with humans, horses can develop skin cancer. When the skin is involved in a cancerous tumor, it is called carcinoma. When the dark pigment cells are involved, the cancerous condition is called melanoma. In horses, melanoma appears to be more prevalent in gray horses because of the dark skin pigmentation. While some forms of skin cancer are benign, melanoma often is malignant.

For years, the treatment of choice for horses suffering from cancer was radical surgery. That has changed, and now horses are treated with both chemotherapy and radiation. One of the drugs of choice in chemotherapy is cisplatin, described as a heavy metal compound that inhibits DNA synthesis by directly binding to DNA, which leads to the death of actively dividing cells.

It has been described as being ideal for treatment of skin cancers where surgery would not be indicated, such as the face, lower extremities, and both male and female genitalia. (For more on cancer in equines see The Horse of June 1996, page 18.)

Many of the skin conditions described above--excluding cancer and alopecia--are preventable through good grooming, sound nutritional and exercise programs, and by maintaining clean equipment and stalls.

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